AP Environmental Science Atmospheric Pollution — Worked Answer Explanations

Unit 7 · 12 questions explained

Below is a complete answer key for our AP Environmental Science Atmospheric Pollution practice questions. For each question you'll find the correct choice, a full written explanation of how to get there, and — for every wrong answer — a short note on exactly why it's tempting and where it goes wrong. Reading these straight through is one of the fastest ways to find the gaps in a unit before exam day.

Prefer to test yourself first? Take the timed Atmospheric Pollution practice test and come back here to review, or head back to the Atmospheric Pollution unit overview.

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Question 1 · Easy
Radon gas is an indoor air pollutant that poses a risk of which disease?
- A
  Asthma and chronic obstructive pulmonary disease (COPD)
  Why not A: Asthma and COPD are associated with particulates, SO₂, and ozone; radon causes cancer, not obstructive lung disease.
- B
  Lung cancer (second leading cause after smoking)Correct
- C
  Mesothelioma (cancer of the lung lining)
  Why not C: Mesothelioma is caused by asbestos fibers, not radon radiation.
- D
  Legionnaires' disease (a bacterial pneumonia)
  Why not D: Legionnaires' disease is a bacterial infection transmitted via water aerosols in HVAC systems; not caused by radon.
Explanation
Radon-222 is a colorless, odorless radioactive gas produced by decay of uranium in soil and rock. It seeps into homes through cracks in foundations. Alpha particles emitted by radon and its decay products damage lung tissue DNA, leading to lung cancer. Radon is the second leading cause of lung cancer in the U.S. (after smoking), responsible for ~21,000 deaths/year. Mitigation: sub-slab depressurization systems.

Key takeaway
Radon: radioactive indoor gas from uranium decay in soil → lung cancer (second leading cause after smoking).
Question 2 · Easy
The difference between tropospheric ozone (bad) and stratospheric ozone (good) is primarily:
- A
  Stratospheric ozone is a different molecule (O₄) from tropospheric ozone (O₃).
  Why not A: Ozone is always O₃ regardless of altitude; it is the same molecule.
- B
  Location: stratospheric ozone absorbs UV; tropospheric ozone is a respiratory irritant and smog component.Correct
- C
  Concentration: stratospheric ozone concentrations are so high they are toxic, while tropospheric ozone is too dilute to matter.
  Why not C: The opposite is true for tropospheric ozone: ambient concentrations (70–150 ppb on smoggy days) are high enough to cause health effects.
- D
  Stratospheric ozone is created by CFCs; tropospheric ozone is created by volcanic activity.
  Why not D: CFCs destroy stratospheric ozone, not create it. Tropospheric ozone forms from NOₓ and VOCs, not volcanoes.
Explanation
Ozone (O₃) is chemically identical regardless of altitude — what matters is WHERE it is. Stratospheric ozone (15–35 km) absorbs UV-B and UV-C radiation, protecting life. Tropospheric ozone is a secondary pollutant formed from NOₓ + VOCs + sunlight; it irritates airways, damages crops and rubber, and is a major component of photochemical smog. 'Good up high, bad down low.'

Key takeaway
'Good up high, bad down low': stratospheric O₃ = UV shield; tropospheric O₃ = respiratory irritant and smog component.
Question 3 · Medium
Photochemical smog forms most intensively on hot, sunny, calm days in urban areas. Which pair of precursor pollutants are MOST directly responsible for initiating the formation of ground-level ozone in smog?
- A
  Sulfur dioxide (SO₂) and particulate matter
  Why not A: SO₂ is a primary pollutant that causes industrial smog (London smog) and acid rain, not photochemical smog.
- B
  Nitrogen oxides (NOₓ) and volatile organic compounds (VOCs)Correct
- C
  Carbon monoxide (CO) and carbon dioxide (CO₂)
  Why not C: CO and CO₂ are not key precursors for photochemical ozone formation; CO is toxic but doesn't form smog through photo-oxidation.
- D
  Chlorofluorocarbons (CFCs) and nitrogen dioxide (NO₂)
  Why not D: CFCs destroy stratospheric ozone; they don't create tropospheric smog. NO₂ is a precursor, but CFCs are not.
Explanation
Photochemical smog formation: NOₓ (from vehicle exhaust and combustion) reacts with VOCs (from gasoline, solvents, paints) in the presence of UV sunlight to form ground-level ozone (O₃) and other oxidants like peroxyacetyl nitrate (PAN). Key conditions: sunshine, heat, stagnant air, and high traffic. The process is driven by photolysis of NO₂: NO₂ + UV → NO + O; O + O₂ → O₃.

Key takeaway
Photochemical smog: NOₓ + VOCs + sunlight → ground-level O₃. LA-type smog. Distinct from London smog (SO₂ + fog).
Question 4 · Medium
Acid rain forms when _____ and _____ react with water vapor in the atmosphere to form sulfuric acid and nitric acid.
- A
  Carbon dioxide (CO₂) and carbon monoxide (CO)
  Why not A: CO₂ + H₂O = carbonic acid (a very weak acid responsible for normal slightly acidic rain at pH ~5.6), not the strong acid rain at pH 4–4.5 caused by SO₂ and NOₓ.
- B
  Sulfur dioxide (SO₂) and nitrogen oxides (NOₓ)Correct
- C
  Chlorofluorocarbons (CFCs) and methane (CH₄)
  Why not C: CFCs deplete stratospheric ozone; methane is a greenhouse gas. Neither reacts with water to form strong acids in the atmosphere.
- D
  Ozone (O₃) and particulate matter (PM₂.₅)
  Why not D: Ozone and particulates are harmful pollutants, but they don't react with water to form the strong acids that cause acid rain.
Explanation
SO₂ (from coal combustion and smelting) + H₂O → H₂SO₄ (sulfuric acid). NOₓ (from combustion in vehicles and power plants) + H₂O → HNO₃ (nitric acid). These reactions lower precipitation pH to 4.0–4.5 (normal rain = pH 5.6). Acid rain damages forests, acidifies lakes (killing fish), corrodes buildings and statues, and leaches toxic metals into soil.

Key takeaway
Acid rain: SO₂ → H₂SO₄; NOₓ → HNO₃. Sources: coal combustion, vehicle exhaust. Effects: forest/lake/building damage.
Question 5 · Medium
The Clean Air Act (CAA) of 1970 and its amendments established National Ambient Air Quality Standards (NAAQS) for six 'criteria pollutants.' Which of the following is correctly identified as a criteria pollutant?
- A
  Carbon dioxide (CO₂)
  Why not A: CO₂ is a greenhouse gas but is NOT a NAAQS criteria pollutant; it is regulated under separate EPA authority (Clean Power Plan).
- B
  Radon (Rn)
  Why not B: Radon is an indoor pollutant managed separately under the Safe Drinking Water Act and EPA voluntary programs; it is not a NAAQS criteria pollutant.
- C
  Ground-level ozone (O₃)Correct
- D
  Chlorofluorocarbons (CFCs)
  Why not D: CFCs are regulated under the Clean Air Act's Title VI (stratospheric ozone protection) but are not NAAQS criteria pollutants.
Explanation
The six NAAQS criteria pollutants are: ground-level ozone (O₃), particulate matter (PM₂.₅ and PM₁₀), carbon monoxide (CO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and lead (Pb). These were chosen because they are widespread, have health effects at ambient concentrations, and come from numerous sources. CO₂ is notably absent — EPA only gained authority to regulate CO₂ under the CAA after the 2007 Massachusetts v. EPA Supreme Court ruling.

Key takeaway
6 NAAQS criteria pollutants: O₃, PM, CO, SO₂, NO₂, Pb. CO₂ is NOT one of them.
Question 6 · Medium
Particulate matter (PM₂.₅) is considered more dangerous to human health than PM₁₀ primarily because:
- A
  PM₂.₅ particles are heavier and deposit more deeply in the lung airways.
  Why not A: PM₂.₅ is smaller (not heavier) than PM₁₀; smaller size allows deeper penetration, but this is not due to greater weight.
- B
  PM₂.₅ particles are small enough to penetrate deep into the alveoli and enter the bloodstream.Correct
- C
  PM₂.₅ contains more sulfur compounds than PM₁₀, making it more toxic.
  Why not C: Both size fractions can contain sulfate compounds; the primary health difference is particle size (penetration depth), not composition.
- D
  PM₂.₅ is only produced by vehicles while PM₁₀ is only from natural sources, making PM₂.₅ more toxic.
  Why not D: Both size fractions come from natural and anthropogenic sources; this framing is incorrect.
Explanation
PM₁₀ (particles <10 µm) is filtered by nasal passages and upper airways. PM₂.₅ (fine particles <2.5 µm, about 1/30th the width of a human hair) can bypass these defenses, penetrate deep into the alveoli, and even enter the bloodstream — causing cardiovascular disease, stroke, and premature death. PM₂.₅ is now considered the most harmful ambient air pollutant by health impact globally.

Key takeaway
PM₂.₅ more dangerous than PM₁₀ because smaller particles penetrate deep into alveoli and enter the bloodstream.
Question 7 · Medium
Indoor air quality is often worse than outdoor air quality in developed countries. Which of the following is a common source of formaldehyde, a significant indoor air pollutant?
- A
  Combustion of natural gas in properly maintained stoves
  Why not A: Properly combusted natural gas produces mainly CO₂ and H₂O; formaldehyde is not a primary product of complete combustion.
- B
  Pressed wood products (particle board, plywood) and foam insulationCorrect
- C
  Drinking water treated with chlorine for disinfection
  Why not C: Chlorination produces disinfection byproducts (trihalomethanes), not formaldehyde; these are water quality issues.
- D
  Lead-based paint in older homes
  Why not D: Lead paint is a serious indoor hazard (heavy metal poisoning) but releases lead dust, not formaldehyde.
Explanation
Formaldehyde (HCHO) off-gasses from urea-formaldehyde resins used in particleboard, MDF, plywood, and foam insulation. It is a known human carcinogen (IARC Group 1) linked to nasopharyngeal cancer and leukemia, and causes acute eye/throat/respiratory irritation. Off-gassing is highest in new buildings with new furniture. Ventilation is the primary mitigation strategy.

Key takeaway
Formaldehyde sources: pressed wood products, new furniture, foam insulation. Carcinogenic; mitigated by ventilation.
Question 8 · Medium
The Montreal Protocol (1987) is widely considered one of the most successful environmental treaties. Its primary achievement was:
- A
  Reducing greenhouse gas emissions to limit global warming below 2°C.
  Why not A: The 2°C climate target is associated with the Paris Agreement (2015), not the Montreal Protocol.
- B
  Phasing out chlorofluorocarbons and other ozone-depleting substances, allowing stratospheric ozone to recover.Correct
- C
  Establishing international standards for acid rain precursor emissions.
  Why not C: Acid rain controls were addressed by national legislation (U.S. Clean Air Act Amendments of 1990) and regional agreements, not the Montreal Protocol.
- D
  Creating the United Nations Environment Programme (UNEP) to govern global pollution treaties.
  Why not D: UNEP was established in 1972 at the Stockholm Conference; the Montreal Protocol (1987) built upon existing institutional structures.
Explanation
The Montreal Protocol required signatory nations to phase out the production and use of chlorofluorocarbons (CFCs), halons, and other ozone-depleting substances. It has been ratified by all 197 UN member states — the first treaty to achieve universal ratification. As a result, stratospheric ozone concentrations have stabilized and the ozone hole is projected to fully recover by ~2065.

Key takeaway
Montreal Protocol: phase-out of CFCs and ozone-depleting substances → stratospheric ozone recovery projected by 2065.
Question 9 · Medium
During a temperature inversion, a layer of warm air traps cooler air (and pollutants) near the ground. This situation most directly worsens which air pollution problem?
- A
  Stratospheric ozone depletion
  Why not A: Temperature inversions are a tropospheric phenomenon; stratospheric ozone depletion is a separate, higher-altitude issue.
- B
  Acid rain deposition hundreds of miles from emission sources
  Why not B: Acid rain involves long-range transport of pollutants aloft, not trapping them near the surface under an inversion.
- C
  Ground-level ozone and photochemical smog concentration in urban areasCorrect
- D
  Radioactive fallout spreading globally after a nuclear event
  Why not D: Radioactive fallout is carried aloft by the explosion; temperature inversions affect near-surface pollution, not upper atmospheric dispersal.
Explanation
Normally, air temperature decreases with altitude, allowing warm polluted surface air to rise and disperse. During a temperature inversion, a warm air layer above traps cooler, denser (and polluted) air below — acting as a lid. Pollutants accumulate. Combined with sunlight and traffic-generated NOₓ and VOCs, inversions intensify photochemical smog. The 1952 London smog event (4,000 deaths) and 1948 Donora, PA event were caused by temperature inversions.

Key takeaway
Temperature inversion: warm air cap traps pollutants near surface → smog intensification. Worse in basins/valleys surrounded by mountains.
Question 10 · Medium
Carbon monoxide (CO) is toxic to humans primarily because it:
- A
  Reacts with lung tissue to form carbonic acid, damaging the airways.
  Why not A: CO does not react with water in lung tissue to form significant acid; that mechanism applies to CO₂.
- B
  Binds to hemoglobin with ~250× greater affinity than oxygen, preventing O₂ transport.Correct
- C
  Interferes with the immune system by mimicking a hormone signal.
  Why not C: CO toxicity is a direct chemical mechanism at hemoglobin; it is not an immune-modulating or hormonal effect.
- D
  Destroys the protective mucous lining of the respiratory tract.
  Why not D: Respiratory tract lining damage is associated with irritants like SO₂ and ozone; CO's toxicity mechanism is systemic, not local airway damage.
Explanation
CO binds to hemoglobin (Hb) at the same sites as O₂, forming carboxyhemoglobin (COHb). CO's affinity for Hb is about 250× greater than O₂, so even low CO concentrations displace oxygen. Cells are starved of O₂ — causing headache, confusion, loss of consciousness, and death at high concentrations. Sources: incomplete combustion (vehicles, furnaces, fireplaces, generators). CO is odorless and colorless — requiring detectors.

Key takeaway
CO binds hemoglobin 250× more strongly than O₂ → carboxyhemoglobin → oxygen starvation → organ damage/death.
Question 11 · Hard
Stratospheric ozone is depleted by chlorofluorocarbons (CFCs). Which chemical process explains how a single CFC molecule can destroy thousands of ozone molecules?
- A
  CFCs directly absorb UV radiation and radiate it harmlessly as infrared, blocking ozone formation.
  Why not A: CFCs don't absorb and re-emit UV to prevent ozone formation; they catalytically destroy ozone that already exists.
- B
  UV radiation breaks CFCs, releasing Cl atoms that catalytically destroy ozone in a chain reaction.Correct
- C
  CFCs react with oxygen to form chlorine dioxide, which blocks ozone from absorbing UV.
  Why not C: The mechanism is catalytic destruction, not blocking; and ClO₂ is not the key intermediate — ClO is.
- D
  CFCs increase tropospheric ozone, which diffuses upward and replaces stratospheric ozone with a different molecule.
  Why not D: Tropospheric ozone cannot diffuse upward in meaningful quantities to replenish stratospheric ozone — the tropopause acts as a boundary.
Explanation
CFC chain reaction: UV radiation in the stratosphere breaks C–Cl bonds → Cl• atom. Cl• + O₃ → ClO + O₂. ClO + O → Cl• + O₂. The Cl• is regenerated and can destroy thousands of ozone molecules before being sequestered. This catalytic cycle was discovered by Molina and Rowland (1974 Nobel Prize) and motivated the Montreal Protocol (1987), which phased out CFCs globally.

Key takeaway
CFC ozone depletion: UV → Cl• + O₃ → ClO → Cl• (regenerated) → chain reaction destroys thousands of O₃ molecules.
Question 12 · Hard
Under the U.S. Clean Air Act's cap-and-trade (emissions trading) system for sulfur dioxide, power plants that reduce SO₂ emissions below their cap can:
- A
  Receive a tax exemption from all future federal environmental regulations.
  Why not A: Cap-and-trade creates tradable permits; it does not exempt firms from regulations.
- B
  Sell their unused emission allowances to other plants that need them, creating economic incentive for reduction.Correct
- C
  Increase their emissions cap in future years as a reward for current reductions.
  Why not C: The cap is set policy-wide and typically decreases over time; individual plant compliance does not increase the cap.
- D
  Apply excess reductions to offset CO₂ emissions under a separate carbon trading scheme.
  Why not D: SO₂ allowances trade separately from CO₂ credits; different pollutants are not directly interchangeable in these systems.
Explanation
The Acid Rain Program (CAA Title IV, 1990) created a cap-and-trade market for SO₂. The EPA sets a total emissions cap and distributes allowances; plants that exceed their cap must buy allowances from plants that reduced below their cap. This market mechanism rewards efficient reducers financially and lets the market find the cheapest reductions — achieving environmental goals at lower total cost than uniform standards. SO₂ emissions from power plants fell >80% by 2015.

Key takeaway
Cap-and-trade: total emissions capped; plants that over-reduce can sell allowances to over-emitters → market finds cheapest emission cuts.